MimeData.ContentType property is what you should use to obtain attachments content type.
Mail.dll doesn't guess the content type, it parses MIME data received from the IMAP or POP3 server.
If the attachment has Content-Type header set to application/octet-stream MimaData.ContentType property will reflect that.
You can use MimaData.SafeFileName to access file name specified by the sender to obtain filename's extension.